There's nothing to stop you doing the ATPL theory course and exams and then doing the IR so that it hangs on your PPL (the ATPL theroetical exams will, I think, count as the IR theory exams for which no one currently runs a course).
However, to hold a Commercial licence and, therefore, to earn money as a pilot, you need a Class 1 medical. If there is any chance you won't be able to achieve a Class 1 then think long and hard about how much you will be spending with, potentially, no reward.
Caveat emptor!